The use of circulators to isolate and transmit electronic signals is well known. Circulators are multi-port devices, which receive a radio frequency (RF) signals on one port and route them to an adjacent port while isolating or decoupling the RF signal from the remaining ports. Currently, circulators are used for applications that operate at very high frequencies. For example, circulators are commonly used in microwave circuits and microwave transmit and receive (T/R) modules for both RADAR and communications systems. Conventional circulator designs may include a y-shaped RF conductor with three port connectors that are positioned between a pair of ferrite substrates. Magnets are placed above and below the ferrite substrates to produce a DC-biasing magnetic field in the ferrite elements to provide non-reciprocal operation of the transmission paths between the three port connectors. A thin metal plate, or cladding, is placed on the outer surface of each ferrite substrate below each magnet to provide ground planes for the circulator and provide shielding from spurious RF radiation. The components are then placed within a steel case or housing to hold provided a return path for the magnetic fields generated by the magnets, while at the same time shielding the components from extraneous magnetic fields.
Although circulators are extremely efficient devices, conventional circulators have several drawbacks. First, installation of conventional circulators on a circuit board requires that an aperture, which is slightly larger than the circulator package is cut into the circuit board where the circulator is to be installed. The circulator is then placed within the aperture and the port connectors are attached to the external circuit trace on the circuit board using manual interconnection, such as solder, ribbon cables, and the like. Since the port connectors of the circulator are normally are made from different materials and have different impedance values from the circuit trace on the circuit board, there is an impedance mismatch at the interconnects, which result is a degradation of the electrical performance of the circulator. The impedance mismatch must be corrected using ribbon connectors, or other known methods to match the impedance the port connectors with the circuit trace. Additionally, discontinuities between the circulator and the circuit trace exist at the connection ports. The manual interconnects also lead to insertion losses at the port connectors, an increase in the interference from unwanted RF signals, and high performance variability of the circulator. Furthermore, the manual interconnects tend to have poor thermal capabilities, which can lead to a decrease in the amount of signal power that can be passed through the circuit.
Another drawback with conventional circulators is that the circulators doe not lie within the same plane as the components of the external circuit. This makes it difficult to effectively provide a common ground the circulator and the circuit. Typically a metal plate must be molded to conform to the contours created by the circulator and adhered to the backside of both the circulator and the external circuit. This non-planar ground plane can lead to reduction in the electrical performance of the circulator.
Yet another drawback to conventional circulators is that they are expensive to manufacture and cannot be made using an automated manufacturing process. For example, the ferrite substrates used in conventional circulators tend to be brittle and can be damaged in an automated manufacturing process. In addition, the components, particularly the resonator, the ferrite elements, and the magnets must be precisely aligned to insure proper operation of the circulator. Consequently, all or at least part of conventional circulators must be assembled manually and the component aligned using a jig or and aligning frame. Once the components are properly aligned, they are sealed, usually by hand, in a steel housing. A spring or other compression mechanism is usually placed in the housing to insure that the ferrite material remains in constant contact with the resonator. Unfortunately, this assembly process is expensive in both time and money.
Several attempts have been made to solve these problems associated with conventional circulators. For example, one method attempted to reduce impedance mismatch between two or more circulator by cascading the circulators in a common package. The circulator includes two or more RF conductors cascaded together, which are disposed between two oblong ferrite substrates. A single impedance matching element is coupled between the coupled connection ports of the cascaded circulator resonators to improve the performance of the circulators. Unfortunately this method still must use manual interconnects to connect the cascaded circulators to an exterior circuit. Furthermore, the circulator elements are disposed between two ferrite substrates, which are easily damaged.
Another solution was to design a cost effective method of manufacturing a large number of circulators. The method includes depositing a circulator trace on a central dielectric substrate. A series of dielectric shims, which are pre-drilled with an opening are disposed around a ferrite element, which rests on top each side of the central substrate. A steel plate is then placed on each side of the substrate layer. An outer shim then is placed on top of the steel disc. The outer shim contains a number of vias etched down to the steel plate to provide an electrical contact to ground. A number of vias are then drilled into the outer shim and filled with a conductive material to provide contacts for surface mounting the circulator to a circuit board. Although the method uses inexpensive materials, this circulator has several drawbacks. First, the steel disc covers only a portion of the circulator trace, which provides an inadequate ground for the circulator trace and consequently does not adequately shield the circulator trace from spurious RF signals. Furthermore, since the circulator is designed for surface mounting, the circulator does not lie in-line with the external circuit and therefore, the ground plane of the circuit is non-planar and discontinuous. The ground plane between the external circuit and the circulator must be bridged with ribbon cables, or other suitable connectors, which results in electrical inefficiencies. Moreover, since the circulator is surface mounted, it uses manual interconnects to connect the circulator to the external circuit, which result in an impedance mismatch between the circulator and the external circuit.
Therefore, there is a need in the art for a low cost circulator that uses standard dielectric materials that can be assembled using conventional printed circuit board (PCB) techniques. There is a further need in the art for a circulator that can be integrated into a circuit, in which the circuit trace of the circulator and the trace of the electrical circuit are part of the same continuous circuit trace without the use of manual interconnects. There is still a further need for a circulator that has a continuous ground plane and can be inserted into a circuit board so that the circulator trace is in-line with the trace of the components from the external circuit.